Aqueous dispersions stabilized from freeze/thaw cycles

ABSTRACT

Aqueous dispersions of PTFE and TiO 2  stabilized from the effects of freeze/thaw cycles are disclosed, wherein the stabilizing agents are monohydric alcohols and monoamines having less than five carbon atoms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to aqueous dispersions of finely divided solidsand, more particularly, to aqueous dispersions of finely divided solidsthat are stabilized against the destructive effects of freeze/thawcycles.

2. Discussion of the Prior Art

It is well recognized that aqueous dispersions of finely divided solidsare somewhat fragile in the sense that care must be taken to maintain astable dispersion in which the particles, depending upon their density,neither settle to the bottom nor float to the top of the aqueous medium.Most generally, a surfactant is used to aid in establishing a stableaqueous dispersion and it is selected to have affinity for the dispersedparticles at one end of its structure and to be hydrophilic at the otherend of its structure. This provides for a basic compatibility betweenthe dispersed particles and the water which, in a sense, can loosely bevisualized as "hydrating" the dispersed particles.

A particularly troublesome problem arises when an aqueous dispersion isshipped or stored during the winter months and the dispersion is exposedto subfreezing temperatures. Generally it may be expected that when anotherwise stable dispersion is exposed to one or more freeze/thawcycles, the dispersion will be "broken". This means that after thefrozen dispersion is thawed, the particles will separate, oftenirreversibly, from the aqueous medium. As will be discussed in moredetail below, it is believed that the reason the dispersion becomesunstable upon freezing is that the freezing process withdraws the "waterof hydration" from the dispersed particles causing them to lose theirmutual repulsion and coalesce.

The state of the art methods used to prevent the destruction of a stabledispersion from freeze/thaw cycles are quite simple. First, it isobvious that the dispersions may be shipped and stored in heatedenvironments to prevent the ambient temperature from falling below thefreezing temperature of the aqueous dispersion. Second, "anti-freeze"agents, such as alcohols and glycols, may be added to the aqueousdispersion in sufficient amounts to depress the freezing point to atemperature below that at which the aqueous dispersion will be exposed.Both of these methods have their drawbacks. For example, maintaining asuitable ambient temperature during shipping and storing requiresspecial handling; it is somewhat expensive; and it is subject to humanerror, as when a container of an aqueous dispersion is accidentally leftstanding on a loading dock in cold weather.

Anti-freeze agents that depress the freezing point of the aqueousdispersion have several disadvantages in that they add an expense; someof them may impart toxicity to the aqueous dispersion; and, ofparticular concern depending upon the nature of the aqueous dispersionwith which they are used, the anti-freeze agents when added insufficient quantities to significantly depress the freezing point, mayadversely effect the stability of the aqueous dispersion. In the latterinstance, the useful properties of the dispersion and its commercialutility may be lost.

Occasional references to other methods for stabilizing an aqueousdispersion from the effects of freeze/thaw cycles can be found in theliterature. For example, U.S. Pat. No. 4,053,443 discloses that a pipejoint sealing compound comprised of an aqueous dispersion offluorocarbon particles can be freeze/thaw stabilized by the addition ofa combination of sodium hydroxide and monoethanolamine. U.S. Pat. No.3,879,302 claims the use of freeze/thaw stabilizing compounds selectedfrom the group comprising silanes and silizanes (sic) to stabilizeaqueous dispersions of polytetrafluoroethylene sealing compositions.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to stabilize aqueousdispersions from the destructive effects of freeze/thaw cycles.

Another object of this invention is to provide a method for protectingaqueous dispersions from the effects of freeze/thaw cycles which doesnot require transporting and storing the aqueous dispersions in heatedspaces. Another object of this invention is to provide a method forprotecting aqueous dispersions from the effects of freeze/thaw cyclesthat is not subject to failure, as by human error.

Another object of this invention is to provide a method for protectingaqueous dispersions from the effects of freeze/thaw cycles that canutilize comparatively inexpensive and nontoxic stabilizing agents.

Another object of this invention is to provide a method for protectingaqueous dispersions from the effects of freeze/thaw cycles that canutilize stabilizing agents in such small amounts that they will notadversely effect the stability of the aqueous dispersion.

Another object of this invention is to provide a method for protectingaqueous dispersions from the effects of freeze/thaw cycles that usessmaller amounts of stabilizing agents than is be required to depress thefreezing point significantly.

Briefly, these and other objects of this invention are achieved byselecting a stabilizing agent for inclusion in an aqueous dispersionwhich will be effective, upon freezing, to cause the development ofanomalies in the ice crystals. These anomalies are believed to begeometrically incompatible with the crystalline structure of ice and, asa result, a fully ordered crystal structure is unable to form within theice.

It has been observed that the stabilizing agents which are effective inthe practice of this invention must be polar compounds capable ofhydrogen bonding with water and, it is believed, capable of formingclathrates with water. Especially useful are the monohydric alcoholshaving less than five carbon atoms and the amine analogs of thealcohols. When these stabilizing agents have more than two carbon atoms,the use of their isomers is preferred to make a shorter and more compactmolecule better suited to the formation of a clathrate hydrate.

MECHANISM OF THE INVENTION

It is recognized that it unnecessary to disclose the theoretical basisof an invention to support a patent but, in this instance, it is thoughtthat an explanation of the mechanism, as best it is understood, will behelpful to a clearer understanding of the invention and the intendedscope of the appended claims. While the mechanism as here proposed isbased, perhaps, only on informed speculation, it has proven quiteconsistent with all of the observed data.

1. The Structure of Water and Ice.

A molecule of water contains two atoms of hydrogen covalently bonded toan atom of oxygen. The three atoms do not lie along a straight line asthe two hydrogen atoms are bent toward each other. Opposite to thehydrogen atoms are two electronic clouds of negative electrification. Asa result, part of the water molecule is negatively charged and part ispositively charged, resulting in a highly polar molecule.

Hydrogen atoms in water molecules are attracted to regions of highelectron density and can form weak linkages, which are referred to ashydrogen bonds. For this reason, the hydrogen atoms of one molecule ofwater are attracted to the electron cloud of an adjacent water moleculeand the water molecules exhibit a strong association for each other. Asthe temperature of water is reduced and the freezing point isapproached, this association becomes highly ordered and, at freezing, acrystal lattice is formed that is made up of the water molecules joinedto one another by the hydrogen bonds. It has been suggested that theoxygen atoms in the ice crystal are situated in "puckered" or "dimpled"layers, and the atoms within each layer are, in turn, arranged in ahexagonal fashion. Thus in ice the collective strength of an essentiallyinfinite network of hydrogen bonds holds the molecules of water in avery specific geometry, dictated by the disposition of the hydrogenatoms, that is two covalently bound and two hydrogen bonded around eachoxygen atom. Substitution of a non-hydrogen bonding impurity moleculeand the introduction of a defect in the crystal does not usually happenbecause of the energetic unfavorability (loss of strong hydrogen bondinginteractions) if it were to happen.

When water that contains other materials such as dissolved solids,liquids, or gases freezes, the solid ice phase that forms is essentiallypure water. Since the chemical similarity between most dispersedparticles and water is not very great, there is very little tendency foran ice crystal to incorporate a dispersed particle "accidentally" at alattice site that "should have" been occupied by a water molecule.

The hydrogen bonding interactions and the highly ordered crystalformation which occurs when water freezes excludes dissolved solids andgases and dispersed solids from the forming ice crystals. Viewed fromanother aspect, the freezing can be considered a "dehydration" processin which the water is removed from the dissolved or dispersed foreignsubstances as the water-to-ice conversion progresses.

Applying the above to the freeze/thaw stabilization of aqueousdispersions, it is generally believed that particles that are held indispersion by surfactants are made stable by "water of hydration", whichis to suggest that the water molecules interact with the polar ends ofthe surfactant molecules and enshroud the otherwise water insolubleparticle. Upon freezing, the "water of hydration" is withdrawn causingthe dispersed particles to contact each other and coalesce. It followsthat when the water is melted, the coalesced particles may precipitatefrom the dispersion in an irreversible manner.

The discovery upon which this invention is based is that the stabilizershere disclosed are effective to inhibit the freezing induced coagulationprocess by preventing, or at least strongly inhibiting, the"dehydration" of the dispersed particles. This results from the factthat the stabilizers interfere with the formation of highly ordered icecrystals and provide protective spaces surrounding the particles toprevent their "dehydration" and subsequent coalescing.

2. The Stabilizing Agents.

The stabilizing agents of this invention are agents are monofunctionalpolar compounds which form a clathrate with water ice crystals (heresometimes refereed to as clathrate hydrates). As is known, a clathratecompound is an inclusion complex in which molecules of one substance arecompletely enclosed within a cage like structure of molecules of anothersubstance. In the case of ice, a clathrate hydrate can be formed inwhich the ice crystal is comprised of perhaps 20 or more water moleculesarranged in an approximately spherical structure surrounding aclathrating molecule. The clathrate hydrate is itself a polar compoundthat can hydrogen bond to the hydrophilic end of a surfactant or,alternatively, to other clathrate hydrates. In the former instance, thedispersed particles are protected from "dehydration". In the latterinstance the clathrate hydrate molecules can bond to themselves, toordinary ice crystals or to the hydrophilic end of the surfactant, anyof which will result in a highly disordered crystalline structure thatwill provide protective spaces encapsulating the dispersed particles.

The clathrating agent must, of course, be a molecule small enough to fitwithin the clathrate hydrate, it must have a polar end group capable ofhydrogen bonding with water and it should have greater affinity forwater than it has for the particles to be dispersed. These requirementsessentially limit the selection of clathrate hydrate formers tomonohydric alcohols and amines that have less than five carbon atoms. Inthe case of the alcohols, the methyl alcohol molecule is slightlysmaller than ideal, the ethyl alcohol molecule approaches the ideal sizeand butyl and propyl alcohol are slightly too large and therefore, ifthey are used to form the clathrate hydrate, it is preferred that theirisomeric forms be used to reduce the length of the carbon chain in anyone direction. The same generalities apply to the amine analogs of thealcohols.

It is of special interest that the clathrating agents need not be usedin large amounts for its only necessary to provide relatively fewclathrate hydrate molecules to accomplish the desired disruption ofordinary ice crystals. As the clathrate hydrate formed around ethylalcohol, for example, is thought to contain about 20 water molecules,the addition of only a 0.05 molar quantity of the clathrating agent willprovide for the theoretical alignment of essentially all of the watermolecules into clathrate hydrates. In point of fact, it has been foundthat only about 5% of theoretical maximum clathrating agent need beused, or about a 0.0025 molar quantity will suffice. Quantities ofclathrating agents this small will not depress the freezing point of aaqueous dispersion to any material degree. By comparison, forty timesthis amount, that is a 0.1 molar quantity of ethyl alcohol, is requiredto depress the freezing point of the aqueous dispersion by ten degreesCelsius.

Experimental data supports the conclusion that the alcohols and amineshere discussed due indeed induce the formation of a clathrate hydrate inwhich the alcohol and amine molecules are enclosed or caged. Since theseclathrating agents fill the free volume of the clathrate hydrate, thevolume occupied by the water and the clathrating agent is less than thetotal volume of the water and the clathrating agents before they aremixed together. As a result, the end point at which the water will notform additional clathrate hydrates can be determined by observing whenthe volume of the aqueous dispersion begins to increase in directproportion to the amount of the clathrating agent added to the aqueousdispersion.

In addition to cost saving, it is sometimes functionally important thatlow levels of a clathrating agent be used. For example, in the case ofan aqueous dispersion of finely divided polytetrafluoroethyleneparticles in a size range of from about 0.2 to 0.5 microns, the additionof sufficient quantities of ethyl alcohol to cause a meaningfulreduction in the freezing point of the aqueous dispersion will result inthe destruction of the dispersion and the precipitation of thepolytetrafluoroethylene particles.

EXAMPLES

Tabularized below, grouped in decreasing order of effectiveness asstabilizing agents to protect against freeze/thaw cycles, are variouscompounds which have been tested. The minimum effective weightpercentages listed are approximations.

The aqueous dispersions used in these examples were comprised of 30% byweight of polytetrafluoroethylene and were prepared from a commerciallyavailable dispersion of polytetrafluoroethylene particles of about 0.5micron or smaller particle sizes as dispersed in water and an non-ionicsurfactant (Sold under the trade designation Teflon (R) B from E. I. DuPont de Nemours and Company.)

The data was obtained by freezing the dispersions at a temperature of-20 degrees Celsius and, after holding them at this temperatureovernight, letting the samples thaw at room temperature. The thawedsamples were evaluated for their effectiveness.

In addition to the data tabulated, some tests were made using finelydivided solids other than polytetrafluoroethylene, including titaniumdioxide and other pigments, and it was found that the stabilizing agentsof this invention had a beneficial effect upon the stability of thesedispersions after exposure to freeze/thaw cycles.

    ______________________________________                                        Minimum Effective                                                             Stabilizing Agent                                                                              Weight Percentage                                                                           Rank                                           ______________________________________                                        Ethyl alcohol    0.6           1                                              Methyl alcohol   0.8           2                                              Isopropyl alcohol                                                                              0.8                                                          Diethylamine     1.0           3                                              Dimethyl sulfoxide                                                                             1.0                                                          Formaldehyde     1.2                                                          Acetone          1.6           4                                              1,4-butanediol   1.8                                                          Diethylene glycol                                                                              1.5                                                          Dimethoxyethane  1.3                                                          Dimethylformamide                                                                              1.4                                                          2-(2-ethoxyethyl)ethanol                                                                       1.3                                                          Ethylene glycol  1.8                                                          Glycerol         1.4                                                          1,6-hexanediamine                                                                              1.8                                                          N-methylpyrrolinone                                                                            1.9                                                          Acetic acid      1.8           5                                              t-Butyl alcohol  1.5                                                          Glucose          2.0                                                          Phosphoric acid  1.9                                                          Sucrose          2.2                                                          Urea             1.6                                                          Acetonitrile                   6                                              2-Butanone                                                                    Formic acid                                                                   s-Butyl alcohol                                                               ______________________________________                                    

I claim:
 1. A method of stabilizing an aqueous dispersion of particleshaving an average size of less than one micron, said particles beingselected from titanium dioxide and polytetraflouroethylene, against theharmful effects of freeze/thaw cycles comprising adding a stabilizingagent to said dispersion wherein said stabilizing agent consistsessentially of an organic, monofunctional, polar compound having lessthan five carbon atoms selected from a monohydric alcohol and amonoamine, and wherein said stabilizing agent is present in an amountnot exceeding 0.05 molar based upon the water present in said aqueousdispersion.
 2. The method of claim 1 wherein said stabilizing agentforms a clathrate hydrate with water.
 3. The method of claim 1 whereinsaid stabilizing agent is used in an amount between about 0.0025 and0.05 molar based upon the water present in said aqueous dispersion. 4.The method of claim 2 wherein said stabilizing agent is ethyl alcohol.5. The method of claim 1 wherein said stabilizing agent is an isomericform of propyl or butyl alcohols.
 6. An aqueous dispersion of particleshaving an average size of less than one micron, said particles beingselected from titanium dioxide and polytetraflouroethylene, that isstabilized against the adverse effects of freeze/thaw cycles by thepresence of a stabilizing agent consisting essentially of amonofunctional polar compound having less than five carbon atomsselected from a monohydric alcohol and a monoamine, said stabilizingagent being present in an amount less than 0.05 molar based upon thewater present in said aqueous dispersion.
 7. The dispersion of claim 6wherein said stabilizing agent forms a clathrate hydrate with water. 8.The dispersion of claim 6 wherein said particles have an average size ofless than about 0.5 micron.
 9. The dispersion of claim 6 including asurfactant.
 10. The dispersion of claim 6 wherein said stabilizing agentis ethyl alcohol.
 11. The dispersion of claim 10 including a surfactant.